The present invention relates to a method for the preparation of an acrylic polymer modified with an organopolysiloxane or, more particularly, to a method for the preparation of an acrylic polymer modified with an organopolysiloxane capable of giving a rubber having remarkably improved heat and cold resistance, oil resistance and mechanical strengths and useful as a material for sealings, O-rings, gaskets, hoses, electric wire sheaths and the like.
Needless to say, organopolysiloxanes in general have excellent heat and cold resistance and weatherability as well as good electric properties so that silicone rubbers formulated with an organopolysiloxane as the base polymer are widely used in a variety of industrial applications. Silicone rubbers are, however, not free from several problems and disadvantages including the relatively poor oil resistance to show considerable swelling in oils even after full curing by crosslinking with an organic peroxide as a curing agent because the principal ingredient of the silicone rubber compounds is a dimethylpolysiloxane which is soluble in many organic solvents including gasoline, rubber solvents and the like. This disadvantageous property of silicone rubbers can be improved by introducing, for example, 3,3,3-trifluoropropyl groups into the organopolysiloxane molecules as the pendant groups to the polysiloxane chain although the practicability of this means is questionable or limited because of the greatly increased costs for the preparation of such a special organopolysiloxane.
On the other hand, acrylic rubbers are promising as a material for automobiles by virtue of their excellent heat resistance and oil resistance but the cold resistance thereof is relatively poor so that it is eagerly desired to develop an acrylic rubber improved in respect of the cold resistance.
Accordingly, several attempts and proposals have been made to combine a silicone rubber and an acrylic rubber each to compensate the defects in the other. For example, Japanese Patent Kokai 55-7814 and 60-152552 have made a proposal to compound an uncrosslinked organopolysiloxane and an uncrosslinked acrylic rubber with a copolymer obtained by the copolymerization of an organopolysiloxane having aliphatically unsaturated hydrocarbon groups and an ester of acrylic acid so as to obtain a rubber blend having good miscibility and workability and capable of being cured with an organic peroxide as the curing agent. Because the copolymer compounded with the organopolysiloxane and acrylic rubber is a product of the copolymerization between the silicon-bonded aliphatically unsaturated groups, e.g., vinyl groups, and the acrylic groups in the acrylic ester, the copolymer is sometimes gelled when the molecular weight thereof is sufficiently high. Moreover, the desired improvement by the addition of this copolymer is obtained only with a large amount of addition thereof while a rubber blend compounded with a large amount of the copolymer has a problem in the behavior of vulcanization not to give a vulcanizate having sufficiently high mechanical strengths because the copolymer is not co-vulcanizable with the other rubbery components by use of an organic peroxide due to the absence of any functional points pertaining to the crosslinking reaction such as aliphatically unsaturated groups.